Acceleration sensor

ABSTRACT

An acceleration sensor includes a body supporting a casing within which a magnetic member is received. A plurality of magnets are mounted on the body and disposed around the casing. The magnetic forces of the magnets hold the magnetic member in a reference position within the casing. The magnetic member is displaceable from the reference position against the magnetic forces of the magnets when the acceleration sensor is subjected to acceleration. A detection device is mounted externally of the casing so as to detect displacement of the magnetic member from the reference position. The acceleration sensor further includes a position adjusting device for adjusting the position of at least one of the magnets relative to the casing so as to set the intensities of the magnetic fields of the magnets in predetermined relation within the casing.

This application is a continuation of U.S. application Ser. No.07/375,886, filed July 6, 1989, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a sensor for detecting acceleration.

2. Prior Art

Japanese Laid-Open (Kokai) Patent Application No. 109374/88, filed bythe Applicant of this application, discloses an acceleration sensorwhich comprises a tubular casing, a magnetic member in the form of aniron ball received within the casing, a differential transformer(detection member) mounted externally of the casing, and a ring-shapedpermanent magnet disposed externally of the differential transformer toattract the iron ball to a central portion of the differentialtransformer. The casing, the differential transformer and the permanentmagnet are mounted within a body immovably relative to the body. Thesensor is mounted on an object to be sensed in such a manner that theaxis of the casing coincides with the sensing direction.

In the above conventional acceleration sensor, particularly, therelation between the magnetic force of the permanent magnet and theposition of the casing, as well as the relation between the magneticforce of the permanent magnet and the position of the differentialtransformer, is important. However, intended functions have not alwaysbeen achieved due to manufacturing errors of the component parts and anerror in magnetization of the permanent magnet.

Japanese Laid-Open Patent Application No. 144261/88 and JapaneseLaid-Open Utility Model Application Nos. 97862/88 and 109654/88, filedby the Applicant of this application, also disclose acceleration sensorsemploying a tubular casing, an iron ball, a permanent magnet and adifferential transformer. Based on the above four earlier Japaneseapplications and other applications, a U.S. patent application was filedon Oct. 23, 1987 under Ser. No. 113,180.

U.S. Pat. Nos. 2,979,959, 4,311,051 and 4,365,513 describe accelerationsensors using an iron ball.

Japanese Laid-Open Patent Application No. 62870/86 discloses atwo-dimensional acceleration sensor comprising a pair of permanentmagnets disposed respectively on opposite sides of a disc-shaped casing.

Further, U.S. Pat. Nos. 3,100,292 and 4,047,439 and Japanese Laid-OpenPatent Application Nos. 203861/85, 233564/85 and 252271/85 discloseacceleration sensors.

However, none of the above-mentioned prior art publications disclosemeans for adjusting the relative position between the permanent magnetand the casing and the relative position between the permanent magnetand the differential transformer.

SUMMARY OF THE INVENTION

It is therefore an object of this invention to provide an accelerationsensor which achieves intended functions without fail.

According to the present invention, there is provided an accelerationsensor comprising:

(a) a body;

(b) a casing supported on the body;

(c) a magnetic member received within the casing;

(d) a plurality of magnets supported on the body and disposed around thecasing, magnetic forces of the magnets holding the magnetic member in areference position within the casing, and the magnetic member beingdisplaceable from the reference position against the magnetic forces ofthe magnets when the acceleration sensor is subjected to acceleration;

(e) detection means mounted externally of the casing so as to detectdisplacement of the magnetic member from the reference position; and

(f) means for adjusting the position of at least one of the magnetsrelative to the casing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of an acceleration sensorprovided in accordance with the present invention; and

FIGS. 2 and 3 are respectively cross-sectional views taken along linesperpendicularly intersecting each other.

DESCRIPTION OF THE PREFERRED EMBODIMENT OF THE INVENTION

A preferred embodiment of the invention will now be described withreference to the drawings

An acceleration sensor shown in FIGS. 1 to 3 broadly comprises a body10, a pair of permanent magnets 21 and 22 supported on the body 10 inopposed relation to each other, a casing 30 received in the body 10, adifferential transformer 50 mounted on the casing 30 and serving as adetection means, a magnetic member 45 in the form of an iron ballreceived within the casing 30, and a pair of covers 71 and 72 fixedlysecured respectively to the upper and lower ends of the body 10.

As shown in FIG. 1, the body 10 has a tubular shape and a rectangularcross-section, the body 10 having a pair of parallel opposed side walls10a and 10b and another pair of parallel opposed side walls 10c and 10ddisposed perpendicular to the side walls 10a and 10b. A hole 11 isformed through each of the four corner portions of the body 10 where theside walls 10a and 10b intersect the side walls 10c and 10d, the hole 11extending between the upper and lower ends of the body 10. Recesses 12and 12 are formed in the upper ends or edges of the side walls 10a and10b, respectively, and two screw holes 13 and 13 are formed in thebottom of each of the two recesses 12 and 12 and are spaced from eachother along the side wall 10a (10b). Two recesses 14 and 14 are formedin the upper end or edge of the side wall 10c and are spaced from eachother along the side wall 10c. A U-shaped notch 15 is formed in the sidewall 10d, the notch 15 opening to the upper end or edge of the side wall10d.

As shown in FIGS. 1 and 2, a ring-shaped support member 20 is fixedlysecured to the inner surface of the side wall 10c of the body 10, andthe permanent magnet 21 is partially received fixedly in a recessdefined by the side wall 10c and the support member 20. The permanentmagnet 21 is of a cylindrical shape, and projects toward the center ofthe body 10 in such a manner that the axis or center line of the magnet21 is disposed perpendicular to the side wall 10c and is disposedcentrally of the height of the body 10.

As shown in FIGS. 1 and 2, the other permanent magnet 22 is supported onthe side wall 10d of the body 10. More specifically, a ring 23constituting part of the body 10 is fixedly secured to the side wall10d. The ring 23 has an annular base portion 23b and a peripheral flange23a formed on the outer periphery of the base portion 23b at one end ofthe base portion 23b. The ring 23 is fixedly mounted on the body 10 insuch a manner that the base portion 23b is snugly fitted in a lowerportion of the notch 15, with the flange 23a held against the outersurface of the side wall 10d. Internal threads 23c are formed on theinner peripheral surface of the base portion 23b of the ring 23, and asupport member 24 having external threads 24 on its outer peripheralsurface is threaded into the annular base 23b, with the external threads24 threadedly engaging the internal threads 23c. A recess 24b is formedin the inner face or side of the support member 24, and the permanentmagnet 22 which is equal in shape and size to the permanent magnet 21 isfixedly fitted in the recess 24b. The permanent magnet 22 is disposedcoaxially with the permanent magnet 21, and the common axis of the twopermanent magnets 21 and 22 is hereinafter referred to as "X-axis". Agroove 24c is formed in the outer face of the support member 24, and byengaging a screw driver in the groove 24c and rotating the screw driver,the support member 24 is moved along the X-axis together with thepermanent magnet 22, thus adjusting the position of the permanent magnet22.

Each of the permanent magnets 21 and 22 is magnetized in the directionof its axis, that is, in the direction of the X-axis, and the opposedend portions of the two magnets are of opposite polarity, as shown inFIG. 2.

As shown in FIGS. 1 to 3, the casing 30 is made of a suitable resin, andhas a hollow cylindrical portion 31, an end wall 32 closing the lowerend of the cylindrical portion 31, first and second flanges 33 and 34formed on the outer periphery of the cylindrical portion 31 at its upperand lower ends, respectively, a third flange 35 formed on the outerperiphery of the cylindrical portion 31 intermediate the first andsecond flanges 33 and 34, an annular portion 36 extending upwardly fromthe upper end of the cylindrical portion 31 in coaxial relation theretoand being greater in diameter than the cylindrical portion 31, and anannular portion 37 extending downwardly from the end wall 32 in coaxialrelation to the cylindrical portion 31. A plurality of grooves 38 areformed in the inner peripheral surface of the cylindrical portion 31 andextend in the axial direction of the casing 30. The grooves 38 serve aspassages for a damper liquid later described.

The upper open end of the cylindrical portion 31 of the casing 30 isclosed by a lid 40 of rubber. The lid 40 has an outer tubular portion40a and an inner tubular portion 40b. A ring 41 is fitted in the outertubular portion 40a to press it against the inner periphery of theannular portion 36, thereby fixing the outer tubular portion 40arelative to the annular portion 36. Also, the inner tubular portion 40bis closed by a ring 42 fitted thereon With this arrangement, a sealedspace 39, which is defined by the cylindrical portion 31, the end wall32 and the lid 40, is provided within the casing 30. The iron ball 45,having a diameter slightly smaller than the inner diameter of thecylindrical portion 31, is received within the sealed space 39, and thedamper liquid is filled in the sealed space 39. Although the gap orspacing between the the iron ball 45 and the inner peripheral surface ofthe casing 30 is actually very small, this gap is shown in FIGS. 2 and 3in an exaggerated manner for illustration purposes. A cap 46 is fittedon the annular portion 36 of the casing 30.

The casing 30 is disposed between the two permanent magnets 21 and 22and is supported by the body 10. More specifically, as shown in FIGS. 1and 3, the first flange 33 of the casing 30 has a pair of oppositeextensions 33a and 33a extending from its central base portion radiallyof the cylindrical portion 31. Each extension 33a has two holes 33b and33b formed therethrough. The lower surface of the extension 33a isstepped to provide a shoulder 33c. The extensions 33a and 33a of thefirst flange 33 rest respectively on the bottoms of the recesses 12 and12 in the body 10 through adjustment shims 60 and 60. The lengths of theextension 33a and the recess 12 in the direction of the X-axis are equalto each other, and therefore the positioning of the casing 30 relativeto the body 10 in the direction of the X-axis is effected uponengagement of each extension 33a in the recess 12. Also, the shoulders33c and 33c of the extensions 33a and 33a are held against the innersurfaces of the side walls 10a and 10b of the body 10, respectively, toposition the casing 30 in a direction perpendicular to the X-axis. As aresult, the axis or center line of the casing 30 substantially coincideswith the axis or center line of the body 10. The axis of the casing 30as well as the axis of the body 10 is indicated by "Y-axis" in FIGS. 2and 3. The two shims 60 and 60 have the same thickness and each have twonotches 60a formed in the inner edge thereof Screws 61 extendrespectively through the holes 33b, formed through the extensions 33a ofthe first flange 33, and the notches 60a in the adjustment shims 60 andare threaded respectively into the screw holes 13 in the body 10.

As shown in FIG. 1, two terminals 47 and three terminals 48 arerespectively formed on and extend outwardly from the opposite edges ofthe first flange 33 of the casing 30 disposed adjacent to the extensions33a. The two terminals 47 are partially received respectively in the tworecesses 14 formed in the body 10. The three terminals 48 are partiallyreceived in the notch 15 formed in the body 10.

The differential transformer 50 is mounted around the cylindricalportion 31 of the casing 30. More specifically, a secondary coil 52 iswound around that portion of the cylindrical portion 31 extendingbetween the third flange 35 and the first flange 33, and a primary coil51 is wound around the secondary coil 52. Similarly, an inner secondarycoil 52 and an outer primary coil 51 are wound around that portion ofthe cylindrical portion 31 extending between the third flange 35 and thesecond flange 34. The two primary coils 51 and 51 are composed of acontinuous single electric wire, and the opposite ends of this electricwire are connected to an oscillator (not shown) through the terminals 47and 47. One ends of the secondary coils 52 and 52 are connected togetherin a differential fashion through the central one of the terminals 48whereas the other ends of the secondary coils are connected to adetection circuit (not shown) through the other two terminals 48.

The center of the differential transformer 50 in the direction of theY-axis is at substantially the same level as the X-axis.

As shown in FIG. 1, a hole 71a is formed through each of the fourcorners of the cover 71, and similarly a hole 72a is formed through eachof the four corners of the cover 72. The covers 71 and 72 are fixedlysecured to the body 10 by bolts 80, passing through the holes 72a, theholes 11 (formed through the body 10) and the holes 71a, and nuts 81threaded respectively on the bolts 80 (see FIG. 3). Notches 71b areformed in the opposed edges of the cover 71 extending in the X-axisdirection, and similarly notches 72b are formed in the opposed edges ofthe cover 72 extending in the X-axis direction. The notches 71b receivethe heads of the screws 61, respectively The covers 71 and 72 haverespective circular central holes 71c and 72c formed therethrough. Thecap 46 for the casing 30 is received in the circular hole 71c of theupper cover 71, and the annular portion 37 of the casing 30 is receivedin the circular hole 72c of the lower cover 72. A pair of rectangularholes 71d are formed through the upper cover 71 and disposedrespectively on the opposite sides of the circular hole 71c in theX-axis direction. Similarly, a pair of rectangular holes 72d are formedthrough the lower cover 72 and disposed respectively on the oppositesides of the circular hole 72c in the X-axis direction. The electricwires connected to the terminals 47 and 48 pass through the rectangularholes 71d of the upper cover 71 and are led to the oscillator and thedetection circuit.

The support members 20 and 24 respectively supporting the pair ofpermanent magnets 21 and 22, the ring 23, the body 10 and the covers 71and 72 are made of iron, and jointly constitute a magnetic circuit forthe permanent magnets 21 and 22. This magnetic circuit is substantiallysymmetrical with respect to a horizontal plane in which the X-axis lies,and is also substantially symmetrical with respect to a vertical planedisposed perpendicular to the sheet of FIG. 2 and including the Y-axistherein. Magnetic lines of force are produced between the two permanentmagnets 21 and 22 and pass through the iron ball 45.

The acceleration sensor of the above construction is mounted on anobject to be sensed in such a manner that the axis (i.e., the Y-axis) ofthe casing 30 extends in the sensing direction.

A high-frequency AC voltage is applied from the above-mentionedoscillator to the primary coils 51 and 51 of the differentialtransformer 50, so that a high-frequency AC voltage, produced in thesecondary coils 52 and 52 as a differential output, is fed to thedetection circuit. In the detection circuit the differential output issubjected to synchronous rectification, and the high-frequencycomponents are removed from the differential output, and a DC voltage ofa predetermined level (i.e., a reference voltage) is applied to thedifferential output to thereby produce a detection output. Thisdetection output contains information indicative of whether thedetection output represents acceleration or deceleration and alsocontains information indicative of the level of such acceleration ordeceleration.

When the acceleration sensor is subjected to no acceleration (that is,the sensor is in a stationary condition or in uniform motion), the ironball 45 is held stationary under the attracting forces of the permanentmagnets 21 and 22. This stationary position is hereinafter referred toas "reference position". In the reference position, the amplitude of thedifferential output from the differential transformer 50 is ideallyzero, and the detection output is equal to the reference voltage.

When the acceleration sensor is in accelerating motion or deceleratingmotion in the direction of the Y-axis, the iron ball 45 is subjected toan inertia force in a direction opposite to the acceleration directionor the deceleration direction, so that the iron ball 45 is moved in theY-axis direction until the inertia force balances the forces exerted bythe permanent magnets 21 and 22 on the iron ball 45 in the Y-axisdirection. The differential transformer 50 feeds a differential outputof an amplitude corresponding to the amount of displacement of the ironball 45 from its reference position in the Y-axis direction, and inaccordance with this differential output, the detection circuit producesa detection voltage deviated from the reference voltage.

Even when the acceleration is zero as described above, the detectionvoltage actually varies slightly from the reference voltage, that is,above or below the reference voltage. The reason why the detectionvoltage is subjected to such variation will now be described. Thediameter of the iron ball 45 is slightly smaller than the inner diameterof the casing 30, and therefore the iron ball 45 is disposed in pointcontact with the inner surface of the casing 30. The iron ball 45undergoes oppositely-directed radial forces exerted respectively by thepermanent magnets 21 and 22, and is held in contact with one of twothose portions of the inner peripheral surface of the casing 30 disposedclose to the permanent magnets 21 and 22, respectively. The force (i.e.,contact force) under which the iron ball 45 is held against the innersurface of the casing 30 is determined by a difference between the aboveradial forces exerted respectively by the permanent magnets 21 and 22.The iron ball 45 is subjected to a frictional resistance force (in theaxial direction of the casing 30) determined by the product of the abovecontact force and the friction coefficient of the inner surface of thecasing 30. Therefore, even when the acceleration is zero, the iron ball45 is caused to stop at a position where the forces exerted by thepermanent magnets 21 and 22 in the axial direction of the casing 30balances the frictional resistance force. Thus, the iron ball 45 doesnot return to the reference position where the value of the differentialtransformer is substantially zero, and the iron ball 45 is displacedslightly from the reference position within a certain range. As aresult, the amplitude of the differential output of the differentialtransformer 50 fails to be accurately zero.

In order to keep variations in the detection output (which variationsoccur when the acceleration is zero as described above) to within anacceptable range, it is necessary that regardless of the position ofcontact of the iron ball 45 with the inner surface of the casing 30, thecontact force, i.e., a difference between the magnetic forces of thepermanent magnets 21 and 22 acting radially of the iron ball 45, shouldbe as small as possible to thereby reduce the frictional resistanceforce acting on the iron ball 45.

To achieve this, the following requirements must be met. Namely, a firstrequirement is that the intensities of the magnetic fields producedrespectively by the permanent magnets 21 and 22 are equal to each otherin the Y-axis (i.e., the axis or center line of the casing 30). A secondrequirement is that a difference between the diameter of the iron ball45 and the inner diameter of the casing 30 should be as small aspossible.

When the first requirement is met, the force of contact of the iron ball45 with that portion of the inner surface of the casing 30 disposedclose to the permanent magnet 21 is equal to the force of contact of theiron ball 45 with that portion of the inner surface of the casing 30disposed close to the other permanent magnet 22. When the first andsecond requirements are met, the forces, exerted respectively by thepermanent magnets 21 and 22 on the iron ball 45 in its radialdirections, mostly cancel each other regardless of whether the iron ball45 is held in contact with one or the other of the above-mentioned twoportions of the inner surfaces of the casing 30 disposed close to thepermanent magnets 21 and 22, respectively. As a result, the iron ball 45is subjected to a small contact force, so that the frictional resistanceis reduced, thereby keeping, to within a small range, a variation in theamount of displacement of the iron ball 45 from the reference positionwhen the acceleration is zero.

The second requirement can be met relatively easily. With theconventional acceleration sensors, although the first requirement is metat the stage of design, this can not actually be easily done due tomanufacturing errors of the component parts and errors in the magneticforces of the permanent magnets 21 and 22. In this embodiment, however,the first requirement can be easily met by rotating the support member24 to finely move the same in the direction of the X-axis to therebyadjust the position of the permanent magnet 22 in the same direction.

The position adjustment of the permanent magnet 22 will now be describedin detail First, after the acceleration sensor is shook with the hand oris subjected to vibration, the acceleration sensor is allowed to bestationary, and then either the amplitude of the differential output ofthe differential transformer 50 or the amount of displacement of thedetection output from the reference voltage is observed. Then, theposition of the permanent magnet 22 in the direction of the X-axis isadjusted. This procedure is repeated until the position of the permanentmagnet 22 is so adjusted as to reduce the above displacement amount to aminimum.

After effecting the above position adjustment, an adhesive or the likemay be applied to the marginal portion of the outer surface of thesupport member 24 and that portion of the internally-threaded portion23c of the ring 23 disposed adjacent to the above marginal portion, soas to prohibit the support member 24 from rotation.

In this embodiment, also, by adjusting the thickness of the adjustmentshims 60 and 60, the position of the casing 30 is adjusted in thedirection of the Y-axis so that the magnetic center of the differentialtransformer 50 can coincide with the reference position of the iron ball45 determined by the permanent magnets 21 and 22. Referring to the term"the magnetic center of the differential transformer 50", when the ironball 45 is positioned at this magnetic center, the differential outputis theoretically zero. With this arrangement, the relation between theacceleration and the detection output is constant.

The manner of adjusting the position of the casing 30 will now bedescribed in detail. There are provided shims 60 of various thicknesses.Among these, shims 60 and 60 of a certain thickness are selected, andthe casing 30 is fixedly secured to the body 10 by the screws 61 throughthe thus selected shims 60 and 60. In this condition, the accelerationsensor is shook and then is allowed to be stationary, and then eitherthe amplitude of the differential output of the differential transformer50 or the amount of displacement of the detection output from thereference voltage is detected. This procedure is repeated until theshims 60 and 60 of the optimum thickness are selected in order to reducethe above displacement amount to a minimum Since the adjustment shims 60and 60 have the notches 60a, the shims can be easily exchanged merely byloosening the screws 61, and therefore this adjustment can be carriedout efficiently.

There are occasions when the magnetic center line of the permanentmagnets 21 and 22 is out of alignment with its geometric axis. In such acase, when the permanent magnet 22 is rotated to be moved in the X-axisdirection to effect its position adjustment, the magnetic center line ofthe permanent magnets 21 and 22 is displaced in the direction of theY-axis, so that the reference position of the iron ball 45 is displacedin the direction of the Y-axis. Therefore, it is preferred that theposition adjustment of the casing 30 in the direction of the Y-axisshould be carried out after the position adjustment of the permanentmagnet 22, because this ensures that the reference position of the ironball 45 can be positively caused to coincide with the magnetic center ofthe differential transformer 50.

In the case where the axis or center line of the casing 30 is disposedvertically, the reference position of the iron ball 45 is displacedslightly below the position at which the iron ball 45 would be locatedby the attracting forces of the permanent magnets 21 and 22 unless theiron ball 45 is not subjected to the influence of gravity. Thisdisplacement of the iron ball 45 due to gravity can be compensated forby the position adjustment of the casing 30. Therefore, regardless ofwhether the axis of the casing 30 is disposed vertically orhorizontally, variations in the detection output can be eliminated.

According to a modified form of the invention, by adjusting the positionof the permanent magnet 22 in the direction of the X-axis, theintensities of the magnetic fields of the permanent magnets 21 and 22may be different from each other by a predetermined amount at the axisor center line of the casing 30. With this arrangement, the iron ball 45is always held against that portion of the inner surface of the casing30 which is disposed close to that permanent magnet having a greatermagnetic force at the axis of the casing 30, thereby stabilizing theiron ball 45. In such a case, it is preferred that the differencebetween the intensities of the magnetic fields of the permanent magnets21 and 22 at the axis or centerline of the casing 30 should be so smallthat the force exerted by one of the two permanent magnets 21 and 22 onthe iron ball 45 in one radial direction of the iron ball 45 can cancel,as much as possible, the force exerted by the other permanent magnet onthe iron ball 45 in the opposite radial direction of the iron ball 45.

While the acceleration sensor according to the present invention hasbeen specifically shown and described herein, the invention itself isnot to be restricted to the exact showing of the drawings or thedescription thereof and various modifications can be made.

For example, the positions of the pair of permanent magnets disposedrespectively on the opposite sides of the iron ball can be bothadjusted. Also, two pairs of permanent magnets can be disposed aroundthe iron ball and be circumferentially spaced from one another at anangle of 90 degrees.

A threaded rod portion may be integrally formed on the support memberfor adjusting the position of the permanent magnet, in which case thethreaded rod portion is threaded into the internally threaded portion ofthe body so as to be suitably rotated to effect the position adjustmentof the permanent magnet, and thereafter the rotation of the supportmember is prohibited by tightening a lock nut threaded on the threadedrod portion.

The magnetic member may be in the form of a rigid cylindrical member ofa magnetic material or in the form of a magnetic fluid comprising acolloid-like liquid having particles of a ferromagnetic materialdispersed in a solvent.

Instead of the transformer, the detection means may comprise capacitorseach mounted on the outside of the casing and having a pair of electrodeplates. In this case, the displacement of the magnetic member isdetected in terms of a variation in electric capacity.

The permanent magnets may be replaced by electromagnets. In this case,the detection output of the detection means may be fed back to a controlcircuit which functions to control the supply voltage of theelectromagnets, thereby controlling the supply voltage of theelectromagnets so that the magnetic member is always located in thereference position. Since the acceleration corresponds to this supplyvoltage, the acceleration is determined by the supply voltage.

The above sensors can be used as sensors for detecting the inclinationof an object.

What is claimed is:
 1. An acceleration sensor comprising:(a) a body; (b)a tubular casing supported on said body; (c) a magnetic member receivedwithin said casing so as to be movable along an axis of said casing; (d)a pair of magnets supported on said body and disposed radiallyexternally of said casing, said casing being interposed between saidpair of magnets, magnetic forces of said magnets holding said magneticmember in a reference position within said casing, and said magneticmember in a reference position within said casing, and said magneticmember being displaceable from said reference position along the axis ofsaid casing against the magnetic forces of said magnets when saidacceleration sensor is subjected to acceleration; (e) detection meansmounted externally of said casing so as to detect displacement of saidmagnetic member from said reference position; and (f) position adjustingmeans for adjusting the position of at least one of said magnets in adirection perpendicular to the axis of said casing, said positionadjusting means comprising a threaded portion formed on said body and asupport member engaging said threaded portion and supporting said atleast one of said magnets.
 2. An acceleration sensor according to claim1, in which the position of said one magnet is so adjusted by saidposition adjusting means that the intensities of the magnetic fields ofsaid pair of magnets are substantially equal to each other at the axisof said casing.
 3. An acceleration sensor according to claim 1, in whichthe position of said one magnet is so adjusted by said positionadjusting means that the intensities of the magnetic fields of said pairof magnets are different from each other by a predetermined amount atthe axis of said casing whereby said magnetic member is always heldagainst that portion of the inner surface of said casing disposed closeto that magnet having a greater magnetic field intensity at the axis ofsaid casing.
 4. An acceleration sensor according to claim 1, in whichthe other of said pair of magnets is fixed relative to said body.
 5. Anacceleration sensor according to claim 1, in which said detection meanscomprises a differential transformer fixedly mounted on said casing.